The present application relates to the general field of paper manufacturing. It relates in particular to metering rod assemblies used in the chemical treatment of paper during various coating processes.
During the coating process, a coating or metering rod assembly lays against a high speed transfer roller. A generous chemical coating is applied to the transfer roller. Excess coating is removed by the metering rod assembly leaving only a thin film of chemical treatment or coating on the transfer roller. Then the coating is transferred to a web. In some instances, an excess amount of coating is applied directly to the web and then the excess is removed with the metering rod.
The metering rod assembly includes a body having a rod channel (rod holder) that is designed to receive a metering rod (sometimes referred to as a doctor blade). The metering rod is generally, but not always, small in diameter and can be very long in length (up to 15 meters). During the coating process, the metering rod primarily controls the coating film thickness. In addition, the rod is rotated within the rod channel to provide uniform wear along the circumference of the rod.
Both metering rods and rod holders are wear parts that have service lives, depending on the application, anywhere from a few hours to a few months. The part of the rod holder that typically wears out is called a cradle and includes the rod channel in which the metering rod rotates. As the metering rod rotates, friction between the metering rod and the cradle, combined with sometimes abrasive coatings and other factors, causes the “lips” of the rod holder cradle to wear down.
In some instances, the cradle is included in an insert that is received in the body of the rod assembly. Such inserts allow replacing the cradle without having to replace the entire body resulting in a cost savings to the customer. Known metering rod assemblies utilizing an insert have generally required replacement of a conventional body with a body designed to accommodate an insert. A body configured to accommodate an insert generally will include one or more features designed to enable clamping the insert in the body to reduce or eliminate vibration and/or unintended separation of the insert from the body. For example, pneumatic clamping tubes are often supported by the body and used for securing an insert in a channel or groove of the body. Such clamping tubes can be pressurized to achieve a variable clamping effect on a metering rod.
While metering rod assemblies utilizing inserts and clamping pressure tubes have met with commercial success, the addition of clamping pressure tubes to the metering rod assemblies complicates the device, increases cost, and can result in an increased risk of operator errors or equipment failure. In addition, upgrading a metering rod assembly to accommodate an insert with the variable clamping features described above typically requires replacing the body of the assembly with a body equipped with a clamping pressure tube, and plumbing pneumatic lines to the body to inflate the pressure tube, and a control unit. This increases costs and complexity of metering rod assemblies.